This invention relates to a novel CRT (cathode-ray tube) having means for suppressing arcing therein; and particularly for suppressing flashovers in the neck of a CRT having a beaded mount assembly.
A color television picture tube is a CRT which comprises an evacuated glass envelope including a viewing window which carries a luminescent viewing screen, and a glass neck which houses an electron-gun mount assembly for producing one or more electron beams for selectively scanning the viewing screen. Each gun comprises a cathode and a plurality of electrodes supported as a unit in spaced tandem relation from at least two elongated, axially-oriented support rods, which are usually in the form of glass beads. The beads have extended surfaces closely spaced from and facing the inner surface of the glass neck. The beads usually extend from the region close to the stem, where the ambient electric fields are small, to the region of the electrode to which the highest operating potential is applied, where the ambient electric fields are high during the operation of the tube. The spaces between the beads and the neck surfaces are channels in which leakage currents may travel from the stem region up to the region of the highest-potential electrode. These leakage currents are associated with blue glow in the neck glass, with charging of the neck surface and with arcing or flashover in the neck. The driving field for these currents is the longitudinal component of the electric field in the channel.
Several expedients have been suggested for blocking or reducing these leakage currents. Coatings on the neck glass are partially effective to prevent arcing but are burned through when arcing does occur. A metal wire or ribbon in the channel (partially or completely around the mount assembly) is also partially effective to reduce arcing because it is often bypassed due to its limited longitudinal extent, because the limited space between the bead and the neck may result in shorting problems, and because there is frequently field emission from the metal structure.